Question

Benzoic acid, C6H5CO2H, is a weak acid (Ka=6.3x10^-5). Calculate the initial concentration (in M) of benzoic acid that is required to produce an aqueous solution of benzoic acid that has a pH of 2.54

Answer 1

C6H5CO2H    <----> C6H5CO2- + H+
C                    0           0   (initial)
C-x                  x           x    (at equilibrium)

given ,
pH = 2.54
-log [H+] = 2.54
[H+] = 2.884*10^-3 M
So, x= 2.884*10^-3 M

Ka = x*x/(C-x)
6.3*10^-5 = (2.884*10^-3)*(2.884*10^-3) / (C-2.884*10^-3)
(C-2.884*10^-3) = 0.132
C = 0.135 M

Answer: 0.135 M